Subjects performed the AX-Continuous Performance Task (35–37). A cue letter was presented for 500 msec, followed by a 3500-msec delay period, and a 500-msec probe letter. Subjects made a target response (right index finger button press) to the probe letter “X” only when it followed the cue letter “A.” All other stimuli required a nontarget response (right middle finger button press), including trials in which X was preceded by any letter other than a (collectively referred to as “B”). Trials with target (AX) cue–probe pairings occurred with 75% frequency, establishing a tendency to make a target response to the X probe. BX trials placed the highest demand on cognitive control because subjects had to overcome the tendency to make a target response to X. Trials in which either the A or B cue was followed by a non-X letter (collectively referred to as Y) were also included.
